The use of flat, multi-wire signal cables is well known in the telephone and electronics industries as a means of electrically interconnecting various kinds of equipment. Among the advantages of flat cable systems are relatively low cost and the ease of gang-terminating the wires of the cable in existing connectors.
In telephone applications, audio frequency cross-talk between circuits must be controlled to very low levels, so that for example a telephone conversation on one line is not heard on another closely spaced line, or interfered with by noise or other signals. In standard round telephone cables, cross-talk is controlled by twisting together the wires of each pair, the various wire pairs being twisted at several different twist periodicities. A discussion of the reduction of cross-talk may be found in "Principles of Electricity Applied to Telephone and Telegraph Work," pages 334-344, published 1961 by the Long Lines Department, American Telephone and Telegraph Company.
There is an important need in several applications for an inexpensive, very thin, flat cable wherein the signal wires are parallel to each other on closely spaced uniform centers for ease of gang-terminating the wires in a connector, yet where the cross-talk is as low or lower than the cross-talk of standard, twisted pair round cable. For example, the telephone keysets in a business office are normally connected to a distribution box by means of 25-pair, band marked distribution (BMD) cable. This kind of cable is standard throughout the telephone industry, is very low in cost and has good cross-talk characteristics throughout the audio and low radio frequency spectrum. However, to terminate BMD cable, each wire, which has its own distinctive color code band, must be visually selected and carefully connected to the respective contact in a distribution box or of a connector, such as the 25-pair "Champ" made by AMP, Inc., or the "Blue Ribbon" made by the Amphenol Co. Needless to say, the labor-cost of terminating BMD cable is appreciable.
Because of the high cost of building construction many office buildings are now being put up without underfloor ducts for telephone cables. As a result the telephone companies have considerable added difficulty in connecting office telephones, and tenants of the building frequently have to put up with unsightly and cumbersome "make-do" wiring running from their telephone keysets to the distribution boxes in the building. One solution to this problem would be to put the telephone wire underneath the office carpeting. However, standard 25-pair BMD cable is four-tenths of an inch thick and is thus not satisfactory for under-carpet installation. An alternative is to use 25 wire pairs twisted and woven or knitted together in a generally flat configuration. But the physical thickness, the cost, and the time of properly terminating the twisted pairs make the use of woven, twisted pair cable not generally acceptable.
Another proposed solution to the problem of obtaining in a flat telephone cable the combined characteristics of low cross-talk, gang-terminability, and minimum thickness is set forth in U.S. Pat. No. 3,764,727 to J. W. Balde of Western Electric Co. In the Balde cable, the conductor pairs are laminated between layers of a thin insulation, such as "Mylar", and are configured in a zig-zag, "pseudotwist" relation for cross-talk control. This cable design gives very good cross-talk control, and gang-terminability of the conductors, but only at selected points along the cable. A modified form of the "pseudotwist" cable is shown in U.S. Pat. No. 3,761,842 to W. B. Gandrud of Bell Telephone Laboratories.
In a recent article by Balde, Delaney and Lahti entitled "Cross-talk Performance of Flat Cable for Telephone Applications", pages 49-58 of Electronic Packaging and Production, for May 1976 the authors give cross-talk measurements comparing the Blade "pseudotwist" cable with woven twisted pair cable, standard twisted pair switchboard cable (BMD), plain flat parallel wire cable, and copper shielded flat cable. The authors comment on page 58 that cross-talk reduction of -85 dB for a 10 ft. cable (or -95 dB for 50 ft.) with voice signals is a reasonable limit for acceptable performance. For the sake of completness, some of the data given in this article, as well as other data published by those skilled in the art, will be incorporated hereinafter to give a quantitative comparison of prior telephone cable performance with that of the cable according to the present invention.
In the above Balde et al article, the authors point out several very large potential market applications which presently exist for multi-wire flat telephone cable. They further point out that costs, including cost per foot of cable, are an important factor in determining whether of not a particular flat cable system will be acceptable. They also show that the cost of terminating wires in a telephone cable (depending on the design) can be a considerable part of the installed cost of a system. The estimate that the initial cost of manufacturing the Balde "pseudotwist" cable to be about 5 cents per conductor foot or $2.50 per cable foot for 25 pairs. This cost it should be noted is more than ten times the cost of standard 25-pair BMD cable.
Still another flat cable design for telephone use is described and claimed in U.S. Pat. No. 3,757,029 to J. Marshall, of Ansley Electronics Company, a subsidiary of the Thomas and Betts Company. In this patent and in a subsequent article entitled "Perforance Characteristics of Jacketed Shielded Flat Cable" pages 235-239, in the NEPCON 1973 Proceedings, September 1973, the inventor describes a laminated flat cable which has closely spaced parallel wires and which achieves good cross-talk control by means of copper shields. However, this cable is relatively thick and difficult to terminate, and its cost per cable foot is much greater than the cost of BMD cable of comparable pair count.
From the above discussion it should be evident that there exists an important need for an improved flat telephone cable which at once combines the characteristics of very low manufacturing cost, close parallel wire spacing and very small thickness, excellent cross-talk control, and gang terminability, as well as other mechanical and handling characteristics needed by the telephone industry. The present invention fulfills this need.